The goal of this research is to investigate the neural basis of lexical effects on the identification of phonetic categories. This will be accomplished by using functional magnetic resonance imaging (fMRI) in normal subjects as they make phonetic judgments to acoustic tokens which vary in both their degree of phonetic ambiguity as well as their lexical status. This research is expected to elucidate two general issues. First, what are the neural mechanisms which underlie perception of phonetic categories, and second, to what extent do top-down effects of lexical status influence those mechanisms. Experiments will use event-related fMRI, which obviates the need to block stimuli, and a sparse-sampling scanning technique, which allows for the presentation of acoustic tokens in relative silence. It is hypothesized that effects of lexical status on judgments of phoneme identity can be seen at perceptual stages of the speech processing stream, supporting an integrative, connectionist model of lexical processing in which upper levels of processing directly influence perception at lower levels. It is also hypothesized that differential patterns of modulation of activation will emerge in the left superior temporal gyrus, the left inferior temporal gyrus and anterior cingulate as a function of both stimulus uncertainty and lexical status. A discovery of brain areas which show modulation of activation due to top-down effects may shed light on the nature of language deficits in aphasic patients who have lesions in these areas.